1. Field of the Invention
The invention relates to the field of power line communications and, in particular, a method and apparatus that provides common mode transmission.
2. Prior Art
Numerous systems are known for providing communications over power lines. Typically, the communications are sent at a substantially higher carrier frequency (e.g. 50 kHz or higher) than is associated with alternating current power (e.g. 60 Hz). There are substantial high frequency noise and interfering signals such as harmonics of the power signal, switching transients, etc. that interfere with the power line communications. Numerous techniques are known for operating in a noisy environment for example, in some cases, the communications messages are repeated to assure transmission, spread spectrum signals are used in other cases, in addition to many other techniques. A system for providing sensing, communications and control over a variety of media, including power lines, is described in U.S. Pat. No. 4,918,690. Another system which utilizes power line communications is commercially available under the trademark "X10".
The present invention uses a common mode choke or inductor for presenting a high impedance to common mode power line communications. Common mode chokes are used in the prior art, however not for this application. They are used for example, to attenuate a common mode signal, such as the switching noise from a switching power supply.
As will be seen, the present invention provides a method and apparatus for better enabling power line communications. The differential mode noise which frequently troubles power line communications is described in conjunction with FIG. 1. The differential mode noise rejection and common mode transmission of the present invention is described in conjunction with the other figures.